The invention relates to a gas turbo-group according to the preamble of claim 1.
The requirements for performance and efficiency of gas turbo-groups are paralleled by rising requirements both for cooling the thermally highly loaded machine components on the one hand, and for the design of the cooling system on the other hand. Sufficient cooling therefore must be ensured in the interest of operational safety. On the other hand, the consumption of cooling air should be reduced as much as possible. EP 62932 suggested cooling the components of a gas turbine with steam in the closed cycle. This requires a relatively complex sealing of the components carrying the cooling steam. At the same time, the components are cooled solely by convection; no use is hereby made of the effect of a cooling film for reducing the introduction of heat. A number of other documents, for example EP 684 369 or EP 995 891 and the corresponding U.S. Pat. No. 6,161,385 suggest the use of steam or of a steam/air mixture for cooling film-cooled components. However, such processes require relatively large amounts of steam that must fulfill high requirements with respect to purity and superheating in order to prevent obstructions of the film cooling holes that often measure only a few tenths of a millimeter across. Even if the required amount and quality of steam is available, the cooling of the gas turbo-group with steam is not inherently ensured, which is in contrast to cooling with compressor extraction air.
Consequently, the cooling with compressor extraction air continues to have a number of solid advantages, whereby the extracted cooling air volume should be minimized in the interest of the efficiency of the working process. As a result, the design of cooling air systems more and more pushes the limits so as to still provide sufficient cooling even at the most unfavorable operating pointxe2x80x94seen from the standpoint of cooling technologyxe2x80x94yet without using too much cooling air. On the one hand, this means a high sensitivity for deviations of the working process from the design point if, for example, as a result of shifting pressure conditions in a machine, the cooling air volumes vary. On the other hand, an overcooling of thermally loaded components results in a series of other operating points so that the potential performance and efficiency levels are not achieved.
It has therefore been repeatedly suggested, for example in EP 1 028 230 to provide variable throttle points in the cooling air path. DE 199 07 907 suggests to directly adjust the pre-pressure of the cooling air with adjustable compressor rotating rows arranged immediately next to an extraction point for the cooling air. While the implementation of these measures is indeed promising, they are by nature very complicated and hardly suitable for retrofitting existing gas turbo-groups. In addition, the installation of moving parts into the cooling system, as disclosed in EP 1 028 230, is associated with the latent risk of an obstruction of the cooling air lines should a mechanical failure of components occur.
The present invention is based on the objective of preventing the disadvantages of the state of the art in cooling a gas turbo-group of the initially mentioned type.
This objective is realized with the entirety of the characteristics of claim 1.
This means that it is the core of the invention to bypass a throttle point in the cooling air channelsxe2x80x94a throttle point known to the expert from the state of the art for adjusting the cooling air mass flow in a defined mannerxe2x80x94and to use suitable means for designing the bypass mass flow in a variable manner, for example, by locating a regulating means in a second flow channel that acts as a bypass; alternatively to a variable throttling of the bypass, means that exert a variable motive force on the bypass mass flow, for example ejectors, also can be provided. These means are realized in a way so as to be accessible to external control interventions. It is useful that the second flow channel has smaller dimensions than the cooling air channel; in particular, the dimensions of the second flow channel are such that the throughput mass flow accounts for no more than 80% of the mass flow of the main cooling air channel. Values ranging from 20% to 50% are found to be favorable; if it is sufficient for the desired variation range, the size of the bypass mass flow also may be designed with values of 10% and less or even only 5% of the main mass flow, or even less. The design mass flow in the bypass hereby must correspond at least to the desired control dynamics. A particularly advantageous aspect of the solution according to the invention is that it is not necessary to design the entire cooling air mass flow as variable, but that only a partial flow must be varied by suitable means. It is hereby furthermore advantageous that, in particular when cooling the stator, this bypass channel can be passed to a point of the gas turbo-group that is accessible from the outside, while the main cooling system is able to extend in a compact manner along the machine. This is an important advantage with respect to maintenance and adjustability; in addition, such a measure is much more suitable for the retrofitting of existing systems than a variable throttling of the entire main cooling air mass flow.
An advantage with respect to operational safety is that the bypass line is provided, upstream from the union with the main cooling air flow, with a retention system that prevents debris from entering and blocking the cooling air channels if the regulating means in the bypass should break.